Ehrlichia chaffeensis is the causative agent of human monocytotropic ehrlichiosis, the most prevalent life-threatening tick-borne zoonotic disease in the United States. E. chaffeensis, an obligately intracellular gram negative bacterium, selectively infects mononuclear phagocytes and evades immune response through undefined mechanisms that are dependent on secreted ehrlichial effector proteins. To further understand the pathobiology of E. chaffeensis the focus of our research is to understand the role of these effectors in mediating and establishing intracellular infection. We recently reported that E. chaffeensis utilizes type 1 secretion (T1S) system to export tandem repeat protein (TRP) effectors that interact with functionally diverse networks of host proteins, through which host cellular functions are modulated. However, the underlying molecular basis for these interactions and the pathways mediated are not defined. Herein, we demonstrate that the E. chaffeensis T1S effector, TRP120, is modified with the small ubiquitin-like modifier (SUMO) by the host pathway. SUMOylation mediates high affinity protein-protein interactions with host proteins encompassing a common SUMO-interacting motif (SIM), and there is an emerging appreciation for the varied cellular functions this modification induces. Eukaryotes utilize SUMO post translational modification to dynamically extend protein function and mediate protein-protein interactions, subcellular localization, gene expression, and protein degradation. Viral and intracellular bacteria effectors are known to modulate host PTM pathways as a means of hijacking host cellular functions to facilitate infection. Our finding that ehrlichial TRPs are substrates of host SUMOylation pathways represents the first description of direct modification of prokaryotic protein by this pathway. We propose ehrlichial T1S effectors are directly SUMO-conjugated as a means of enabling host protein interactions, dynamic regulation of effector subcellular localization, and modulation of host cell functions that support intracellular infectio and survival. The long-term goal of these studies is to characterize the role of T1S effectors in ehrlichial pathobiology and the role of PTMs in effector function. The objective of this proposal i to demonstrate SUMO-conjugation of ehrlichial TRP effectors by host PTM pathways, which serves as the molecular basis for interactions with diverse host protein that facilitate infection and intracellular survival. In aim 1 we will demonstrate ehrlichial TRPs are directly SUMOconjugated, and molecularly-define the basis for this modification. These studies will provide the tools necessary for aim 2, in which we propose to evaluate the functional impact of effector SUMOylation in the context of E. chaffeensis infection. In total, this research will characterize the roles of these effectors and PTMs in the pathobiology of Ehrlichia, and provide insight into a novel mechanism utilized by intracellular microbes to modulate host cell functions and establish infection.